Effect of long-term service exposure at elevated temperature on microstructural changes of 5Cr-0.5Mo steels
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I.
INTRODUCTION
A 5Cr-0.5Mo steel is widely used in petrochemical industries, primarily because of its high strength and corrosion resistance against oils and crudes containing hydrogen sulfide and other corrosive agents.[1,2,3] This steel is conventionally given an austenitizing treatment in the temperature range of 925 7C to 970 7C followed by either air cooling or furnace cooling and subsequently tempering in the range of 650 7C to 700 7C, which is considerably higher than the maximum operating temperature. Due to the long-term service exposure of this steel at elevated temperature, microstructural changes in terms of carbide coarsening, formation of more stable carbides either from solid solution or at the expense of the existing carbides, and precipitate-free zone formation are expected to occur. The microstructural changes due to long-term exposure might cause mechanical property degradation. In recent years, there has been considerable interest paid by the metallurgists and engineers in extending the service period of a component used at elevated temperatures in power plants, petrochemical industries, chemical and fertilizer plants, etc., beyond its design life due to current economic considerations.[4–8] Among the various methods developed so far, the metallographic method appears to be the most attractive method. Because it requires a small sample, the method could be made nondestructive; in principle, the metallographic method measures the damage micromechanism directly.[9] Therefore, for the estimation of the service life of a component by means of the metallographic S. DAS and A. JOARDER, Scientists, are with the Materials Characterisation Division, National Metallurgical Laboratory, Jamshedpur 831 007, India. Manuscript submitted March 15, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
method, microstructural information of long-term serviceexposed material is required. Several reports and technical articles have already been published on microstructural changes due to long-term service exposure of 1Cr0.5Mo,[10–14] 0.5Cr-0.5Mo-0.25V,[13,15–17] 2.25Cr-1Mo,[18,19,20] and 1Cr-1Mo-0.25V[21–24] steels. However, information relating to microstructural changes due to long-term service exposure of 5Cr-0.5Mo steel at elevated temperatures is scarce and sparsely distributed in the literature. Therefore, the present work was undertaken to strengthen the microstructural information in the literature for 5Cr0.5Mo steel subjected to long-term service exposure at elevated temperatures. Such microstructural information could be helpful for assessing the service life of a component beyond its design life. II.
EXPERIMENTAL PROCEDURE
The steel samples used for the present investigation are commercial grade 5Cr-0.5Mo steels. The chemical composition in weight percent of the steels is given in Table I. The heat treatments were performed by the tube manufacturers and are unknown to the authors. The samples were collected from virgin and service-exposed process heater tubes used in oil refineries. The insides of the t
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